The present invention relates to a plant for transmitting electric power comprising a direct voltage network for High Voltage Direct Current (HVDC) and at least two stations connected thereto with line commutated valves with thyristors as rectifying semiconductor components for transmitting electric power between the direct voltage network and alternating voltage networks connected to the stations, in which it comprises at least a cable having an insulating mantle with a polymer base surrounding the conductor thereof for forming the direct voltage network interconnecting the stations, as well as a method for changing the power feeding direction of a plant of this type according to the preamble of the appended independent method claim.
An advantage of the use of such a cable for power transmission between stations of such a plant is that it is considerably less costly than cables used before with an inner conductor surrounded by a thick insulating mantle formed by paper soak in oil. It has neither the serious disadvantage of a comparatively large disturbing influence upon nature and living creatures as have air lines where they are arranged. However, cables of the type defined in the introduction, which are generally called the PEX-type by the man skilled in the art, are associated with the disadvantage of the known plants of the type defined in the introduction, that they do not allow that the polarity of the cable is changed rapidly, but the cable has to first be discharged, which is a very time consuming procedure. This is a great disadvantage of these plants already known, since the power feeding direction on the direct voltage network between the stations thereof is changed by changing the voltage polarity of the valves and by that of the entire cable. It is necessary to proceed in this way, since the valves can only conduct current in one predetermined direction therethrough. During the time of discharging of the cable no power may be transmitted between the stations, wouldn""t the plant have been provided with an additional extra cable to be switched in. However, it will be unacceptably expensive to have such double cables and the main idea of using this type of cables will then get lost. But there is in many cases a requirement of a so-called emergency power regulation, which means that it should be possible to change the power feeding direction very rapidly for stabilising an alternating voltage network connected to any station belonging to the plant.
The construction of such a plant already known is schematically illustrated in FIG. 1, and this plant has a direct voltage network 1 with a cable 2 schematically indicated having an insulating mantle with a polymer base surrounding the conductor thereof. The plant has two stations 3, 4 which are illustrated by the symbol of a current valve 5, 6 each, which they have and which connect the respective station with an alternating voltage network 7, 8 through a transformer 9, 10 for feeding electric power between the respective alternating voltage network and the direct voltage network. Since the rectifying thyristors arranged in the valves 5 and 6 only are able to conduct current in one direction therethrough, each valve may also only conduct current in one direction between the respective alternating voltage network and the direct voltage network. More exactly, the current direction through the direct voltage cable 2 will always be from the station 3 to the station 4, and the power feeding direction can only be changed by changing the polarity at the stations with a maintained current direction. This requires as mentioned above a long discharging time, during which no power may be transmitted between the stations.
The object of the present invention is to provide a plant of the type defined in the introduction, which makes the so-called emergency power regulation mentioned above possible without any requirement of costly arrangements in the form of extra cables or the like, as well as a method for rapidly changing the power feeding direction according to the type defined in the introduction.
This object is according to the invention obtained by the fact that in a plant defined in the introduction the thyristors of the valves of the station are bidirectionally controlled thyristors (BCT) with ability to conduct current in both directions therethrough, and the plant has a unit adapted to control the thyristors of the valves for changing power feeding direction between the stations by changing the current direction in said cable.
By the new approach to arrange so-called BCTs (Bidirectionally Controlled Thyristors) in valves of a plant of this type the less costly cable of PEX-type may be used for direct voltage networks, but the requirements of the customers of a rapid reversing of the power feeding direction as well as high demands on availability may be fulfilled. No time consuming discharging of the cable is namely required before the power feeding direction may be changed, when this change takes place with maintained polarity and is only achieved through reversing the current direction through the cable. The power feeding direction may in a plant of this type frankly be changed by gradually reducing the power fed and by that the current in the cable in a normal way to zero, or close to zero, so that the transmission through the direct voltage network is temporarily stopped, whereupon a changed current direction is ordered through said unit, whereupon the current is gradually increased again in the opposite direction so as to obtain a level corresponding to a desired level of the power supply in the other direction. The power feeding direction has in this way been changed without changing the voltage polarity of the cable.
The arrangement of BCTs in the valves instead of conventional thyristors, which can only conduct current in one direction, does not result in any considerable extra costs, since they may be built in into the valves on the same places as the conventional thyristors would otherwise have, and no increased insulation distances or large modifications of the control equipment of the stations are required. These bidirectionally controlled thyristors are thyristors of the type described in for example DE 44 39 012 A1 and are sometimes also called two-direction thyristors.
The method according to the invention according to the appended independent method claim is directed to the advantageous way just described to take care of a change of the power feeding direction of a plant of this type, and the advantages of this method, which enables a very rapid reversing of the power feeding, appear very clearly from the discussion above.
Further advantages as well as advantageous features of the invention appear from the following description and the other dependent claims.